Osteoporosis is a silent disease process that takes an enormous medical and economic tool on an aging population. This disease is characterized by low bone mass and microarchitectural deterioration of bone tissue, leading to bond fragility and an increase in the risk of fracture. Currently, osteoporosis affects an enormous number of people, and its prevalence will increase as the population ages. It is estimated that between 13% and 18% of postmenopausal Caucasian women in the United States, approximately 4 to 6 million, have osteoporosis, and an additional 30% to 50% (13 to 17 million) have low bone density at the hip. Based on data from the National health and Nutrition Examination Survey III (NHANES), the National Osteoporosis foundation has estimated that more than 10 million people in the United States have osteoporosis of the hip, and nearly 19 million more have low hip bone mass, placing them at increased risk for osteoporosis and fractures.
In addition to becoming a national health concern, osteoporotic fractures have created a heavy economic burden. In 1995, they were the presumed cause of 432,000 hospital admissions, almost 2.5 million physician visits and approximately 180,000 nursing home admissions in the United States that year. Direct medical expenditures alone for osteoporotic fractures in that year were estimated at $13.8 billion. These costs are anticipated to rise along with the growing elderly population. Hip fractures incur the greatest osteoporosis-related health care expenditures. by one estimate, the number of hip fractures and their associated costs could more than triple by the year 2040.
Fortunately, osteoporosis is preventable and treatable. Unfortunately, because there are no warning signs until fracture occurs, few people are currently being diagnosed in time to receive effective therapy during the disease's early phase. A Gallup survey commissioned by the NOF in 1991 found that three-fourths of all women aged 45 to 75, the group at highest risk, had never even discussed osteoporosis with their physician. There are many risk factors associated with the pathogenesis of osteoporotic fractures, including, but certainly not limited to, age, menopausal status, medications, and nutrition. In general, if one or more risk factors are present, a bone mineral density (BMD) test is appropriate.
Since its commercial inception in 1987, dual X-ray absorptiometry (DXA) has become the most widespread method for assessing a person's bone mineral status. M. Jergas et al., Spinal and Femoral DXA for the Assessment of Spinal Osteoporisis, Calcified Tissue International; Springer Verlag, New York, 1997, 61:351–357. Bone densitometry focuses on the bone mineral area density (BMD in g/cm2) of the proximal femur and spine in anterior-posterior projections. H. Franck et al., Bone Mineral Density of Opposing Hips Using Dual Energy X-ray Absorptionmetry in Single-Beam and Fan-Beam Design, Calcified Tissue International; Springer Verlag, New York, 1997, 61:445–447. However, because the hip has a relatively complex architecture, hip measurements are more demanding for the technician examining this site. Positioning of the hip is error-prone and the evaluation of serial scans may be affected by relatively small changes in rotation and abduction of the hip. Thus, the reproducibility of the hip BMD is generally poorer than that of the spinal BMD. M. Jergas et al., ibid. This poor reproducibility is both clinically and financially important, prevention of osteoporotic fractures, with the attendant reduction in health care costs, depends on early identification of individuals at risk for fracture. To detect these individuals earlier it is absolutely imperative that the accuracy of the BMD increases. In general, a practical clinical guideline is that a measured change in bone density should be equal to or greater than 2.8 times the precision error (coefficient of variation) of the measured technique in an individual patient to be considered “real” as compared to “artificial”. Even with strict compliance to calibration and quality control procedures, the precision error of DXA measurements of the femoral neck is 2% to 3%, indicating that a change of at least 5.6% to 8.4% is needed to be considered real. L. F. Verheij et al., Optimization of Follow-up Measurements of Bone Mass. Journal of Nuclear Medicine; 1992, 33:1406–1410; C. Christiansen, Postmenopausal Bone Loss and the Risk of Osteoporosis, Osteoporosis International; 1994, 9(SI):S47–S51. However, strict compliance to calibration and quality control procedures is paramount to competent testing and therefore, one would expect much higher values clinically in less controlled environments.
Despite the obvious necessity for accuracy, few measures are currently taken to ensure reproducitility. Hologic, Inc., the United States leader in developing and manufacturing DXA systems, recognizes the importance and difficulty in achieving effective repositioning of the hip when performing serial scans. According to the company's QDR 4500 Operator's Manual, DXA machines are equipped with a “Hip Scan Positioning Fixture.” In essence, this apparatus is simply a device to secure the foot following the manual rotation of the leg by the technician. According to the operator's manual, “the patient's foot should be placed in the hip scan positioning fixture. The fixture should then be aligned with the patient's leg, patient's leg rotated by turning the leg and foot, and the foot then placed against the positioning fixture. The strap is adjusted to snugly hold the foot, on the side to be examined, in the correct position.” While this method provides adequate separation between the ischium and the femoral neck for the analysis of the scan, it should be obvious that it does little to enhance the reproducibility of serial scans. Unfortunately, this method does not provide any means of standardizing hip positioning and does not remove the operator-introduced error associated with repositioning.
In a variation of the aforementioned technology, Lunar Corporation published results for a dual-femur leg positioner and associated software. R. G. Mazess et al., Bilateral Measurement of Femoral Bone Mineral Density, J. Clin Den 2000; 3(2): 133–140. The research reported that the use of a dual-femur leg positioner and software allowed the simultaneous bilateral measurement of femoral bone mineral density and thus helped eliminate the error associated with the repositioning needed during single femur scans. However, the device has not eliminated the inherent flaws associated with the similar Hologic “Hip Scan Positioning Fixture”, especially as it pertains to scans taken over several years, which are often most critical to the diagnosis and prevention of low bone mass diseases.
In another application to enhance reproducibility of hip positioning, U.S. Pat. No. 5,522,792, discloses an apparatus for maintaining an immobilized position for DXA scans. This invention immobilizes a patient's legs in a fully-extended, abducted position, while holding the patient's feet in adducted position. The present invention allows a patient to be immobilized by providing a frame on which a patient is placed, a centering member, a pair of knee restraints to secure the knees in a fully-extended and abducted position, and a pair of foot restraints to secure the patient's feet in an adducted position. While it is acknowledged that this device may help reduce the variation in hip positioning, there are several issues, which may limit the use of the device clinically. First, the hip positioning system, or HPS, provides only one single rotation angle because of the rigid nature of the design. Although this was able to optimize the projected length of the femoral neck, due to anatomical variation, no single rotation angle will result in optimal projection for all patients. Second, it has been found that subjects of short stature cannot be properly evaluated using the HPS. D. Hans et al., Effects of a New Positioner on the Precision of Hip Bone Mineral Density Measurements, J. Bone Miner Res 1997; 12(8): 1289–1294.
A report in Calcified Tissue International, 1995, describes a custom-designed positioning jig intended to minimize rotation of the hips during a BMD scan. An ankle foot orthosis (AFO) attached to the jig encased the foot and ankle while straps around the mid-thigh were intended to minimize any movement. The feet were placed 30 cm apart and the legs were internally rotated at 15° angles. This apparatus also presents several possible problems. First, according to Hologic and Lunar, the primary manufacturers of DXA equipment, proper femoral orientation is achieved with a rotation of 25°, 10° more than the angle recommended for optimal reproducibility when using the ankle foot orthosis and attached jig. And second, the jig initiates rotation at the foot or ankle; however, there is no means to assure that any rotation at the angle will correspond directly to a proportional change in angle at the hip. Many medical and physiological instances are foreseeable in which rotation of the ankle would product either little, or exaggerated movements, of the hip, dependent upon the condition.
One contrivance, which has attempted to address this issue, is the Norland “Advanced Hip Positioning Aid.” Unlike many of the existing technologies, the Norland device attempts to rotate the femur by rotating the mid-thigh, rather than by rotating the feet. In vivo results indicate a significant improvement in precision using the new hip positional aid. However, the device still lacks features and considerations, which would promote its use in the clinical environment.
None of the foregoing prior art references provide a device which is simple to use, which would promote accurate, reproducible results regardless of the DXA machine used, and which could be used to measure either femur regardless of the characteristics of the patient to be evaluated.
Because the hip has a relatively complex architecture, hip measurements are more demanding for the technician examining this site. Positioning of the hip is error-prone, and the evaluation of serial scans may be affected by relatively small changes in rotation and abduction of the hip. Thus, the reproducibility of the hip BMD is generally poorer than that of the spinal BMD. Currently, few measures are taken to ensure reproducibility. One method is to strap the foot into a foot brace, the leg being rotated inwards and abducted from the midline. This provides adequate separation between the ischium and the femoral neck for the analysis of the scan but does not quantitate the degree of movement at the hip site. The second quality assurance measure involves having serial BMD's performed by the same technician on the same machine, however this has its obvious limitations and does not remove the human error.